Establishing the role of Br?nsted acidity and porosity for the catalytic etherification of glycerol with tert-butanol by modifying zeolites

Article abstract of DOI:10.1016/j.apcata.2012.10.028

The role of Br?nsted acidity and porosity for the etherification of glycerol with tert-butanol was studied by modifying the surface and acidic characteristics of three commercial Na-zeolites (mordenite, beta and ZSM-5) by protonation, dealumination, desil

Full text of DOI:10.1016/j.apcata.2012.10.028

Applied Catalysis A: General 450 (2013) 178–188
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Applied Catalysis A: General
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Establishing the role of Brønsted acidity and porosity for the catalytic
etherification of glycerol with tert-butanol by modifying zeolites
María Dolores González, Yolanda Cesteros∗, Pilar Salagre
Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, C/Marcel lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The role of Brønsted acidity and porosity for the etherification of glycerol with tert-butanol was studied
by modifying the surface and acidic characteristics of three commercial Na-zeolites (mordenite, beta and
ZSM-5) by protonation, dealumination, desilication-protonation, lanthanum-exchange and fluorination.
Catalytic results can be related to the amount and strength of Brønsted acid sites together with the acces-
sibility of the reactants to the acid sites. Modifications made on ZSM-5 did not affect considerably its
surface and acidic properties, and consequently its catalytic behaviour, leading moderate conversion val-
ues and low/null selectivity to di- and tri-tertiary butyl ethers of glycerol (h-GTBE). Mordenite catalysts
showed low conversion and moderate/low selectivity to h-GTBE due to the lower amount of acid centres,
lower external surface area, more hydrophilic character and lower dimensionality of the mordenite struc-
ture when compared with the other two zeolites. Beta catalysts exhibited the best catalytic results. The
introduction of fluorine in the beta zeolite framework generated higher amounts of stronger acid sites,
which were able to transform glycerol until the glycerol triether. Thus, fluorinated beta yielded the best
conversion (75%) and selectivity to h-GTBE (37%) with the formation of glycerol triether in low amounts.
These values were comparable to those obtained at the same reaction conditions with an Amberlyst-15,
an acid catalyst traditionally used for this reaction.
Received 16 March 2012
Received in revised form
28 September 2012
Accepted 11 October 2012
Available online 7 November 2012
Keywords:
Glycerol etherification
Zeolite
Fluorination
Desilication
Dealumination
Tert-butanol
Brønsted acid sites
Porosity
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
because of their antidetonant and octane-improving properties, but
is detrimental to the environment.
Glycerine (glycerol or 1,2,3-propanetriol) has over 1500 known
end uses, including applications in cosmetics, pharmaceuticals and
food products [1,2]. During biodiesel manufacture, by transesteri-
fication of vegetable oils with methanol, glycerine is formed as
by-product (10 wt.% of the total product) [1–3]. The price of glyc-
erol is falling as fast as biodiesel plants are being built. Research
is currently starting to find new outlets to convert the surplus of
glycerol into high-added value products that improve the economy
of the whole process [1,4–8].
One challenging option is the catalytic etherification of glyc-
erol with tert-butanol or isobutene to obtain di- and tri-tertiary
butyl ethers of glycerol (h-GTBE), which is an excellent addi-
tive with a large potential for diesel and biodiesel reformulation
[9–11]. Thus, when h-GTBE was incorporated in standard 30–40%
aromatic-containing diesel fuel, emissions of particulate mat-
ter, hydrocarbons, carbon monoxide, and unregulated aldehydes
decreased significantly [10,11]. Besides, h-GTBE can replace methyl
tertiary butyl ether (MTBE), which is used as valuable additive
Etherification of glycerol with isobutylene (IB) or with tertiary
butanol (TBA) has been studied in the presence of acid catalysts
[12–21]. Etherification with isobutene yielded to better conver-
sion and better selectivity values to h-GTBE than etherification
with tert-butanol [14,16]. The water formed when using TBA as
reagent seems to have an inhibition effect on glycerol terbuty-
lation. However, the use of tert-butanol, as both reactant and
glycerol solvent, instead of gaseous isobutylene, overcomes the
technological problems arising from the need to use solvents able
to dissolve glycerol (i.e. dioxane, dimethyl sulfoxide) and typi-
cal drawbacks of a complex three-phase system (mass transfer
phenomena) [14,22]. Also, the simpler reaction system with tert-
butanol can be very useful to the systematic study of new catalytic
systems.
There are few studies about the catalytic etherification of glyc-
erol with tert-butanol [13,14,23–25]. Klepácˇová et al. reported that
acidic resin catalysts (Amberlyst type) exhibited higher conversion
(88%) than zeolites H-Y and H-Beta but, on the whole, low selectiv-
ity to h-GTBE (around 25%) was achieved for this reaction, which
were performed in a batch reactor at 338 K with a TBA/glycerol
ratio of 4, at reaction times of 300 min [13,14]. It is important to
note that glycerol triether was not detected when using these H-
zeolites. This has been attributed to steric hindrance effects because
∗
Corresponding author. Tel.: +34 977558785; fax: +34 977559563.
E-mail addresses: yolanda.cesteros@urv.cat, yolanda.cesteros@urv.net
(Y. Cesteros).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.10.028

M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
179
of the microporosity of the zeolites [14]. Luque et al. evaluated
2.2. Catalysts characterization
a new family of mesoporous carbonaceous materials, denoted as
Starbon, as catalysts for this reaction. Etherification reaction was
carried out in a microwave-irradiated tube under continuous stir-
ring for short time periods. The authors reported a conversion of
66% with almost total selectivity to the monoether [23]. Frusteri
et al. did not improve the catalytic results of Amberlyst when using
lab-made silica supported acid catalysts [24]. More recently, Ozbay
et al. studied this reaction in a flow reactor at short residence time
using Amberlyst, Nafion and alumina catalysts [25]. Again, the best
results were achieved with an Amberlyst catalyst but with lower
conversion and selectivity values to h-GTBE than those obtained in
a batch reactor.
The aim of this work was to explore the influence of the amount,
strength and accessibility of Brønsted acid sites on the conversion
and selectivity to h-GTBE for the catalytic etherification of glycerol
with tert-butanol by modifying the acidic and porosity charac-
teristics of three pentasil-type zeolites by different treatment
procedures: protonation, dealumination, desilication-protonation,
lanthanum exchange and fluorination.
Elemental analyses of the samples were obtained with a Philips
PW-2400 sequential XRF analyser with Phillips Super Q software.
All measures were made in triplicate.
Structural characterization was completed by powder X-ray
diffraction patterns of the samples, which were obtained with a
Siemens D5000 diffractometer using nickel-filtered Cu K␣ radi-
ation. Samples were dusted on double-sided sticky tape and
mounted on glass microscope slides. The patterns were recorded
over a range of 2Â angles from 5◦ to 40◦ and crystalline phases
were identified using the Joint Committee on Powder Diffraction
Standards (JCPDS) files (43-0171, 48-0074, 37-359 corresponds to
mordenite, beta and ZSM-5, respectively). Crystallinity of the mod-
ified mordenites was determined by comparing the sum of the peak
areas of (1 5 0), (2 0 2), (3 5 0) and (4 0 2) (22–32◦ 2Â) with respect
to commercial Na-mordenite. Crystallinity of the modified ZSM-5
samples was calculated using the (0 5 1) peak intensity compared
with the parent zeolite sample. The integrated intensity of the
signal at 2Â = 22.4◦ was used to evaluate the crystallinity of beta
samples.
Textural characterization of the solids was performed by
N2 (ꢀN2 = 0.162 nm2) adsorption–desorption at 77 K using
a
2. Experimental
Micromeritics ASAP 2000 surface analyser. Before measurements
all samples were outgassed at 573 K for 6 h. BET surface areas were
calculated using adsorption data in the relative pressure range
0 < P/P0 < 0.1. Micropore and external surface areas were obtained
2.1. Catalysts preparation
Three commercial zeolites were modified by protonation, dea-
lumination, desilication-protonation, lanthanum exchange and
fluorination. Na-mordenite (Zeolyst, Si/Al = 6.5, CBV 10A Lot No.
1822-50), Na-Beta (Zeochem, Si/Al = 10, PB Lot No. 6000186) and
Na-ZSM-5 (Zeochem, Si/Al = 20, PZ-2/40 Lot No. 6002827,01) were
designated as M, B and Z, respectively.
Commercial zeolites were treated with NH4NO3 1 M at 373 K
for 1 h. Samples were washed several times with deionised water
and calcined at 813 K for 5 h to obtain the corresponding H-zeolites
(HM, HB and HZ). Dealuminated zeolites were prepared from com-
mercial Na-zeolites by refluxing with HCl 6 M at 373 K for 2 h (DAM,
DAB and DAZ, respectively).
Samples DSHM, DSHB and DSHZ were obtained by desilication
of commercial zeolites with NaOH 0.2 M under refluxing at 338 K
for 30 min. After treatment, samples were washed until pH 7, dried,
exchanged with NH4NO3 1 M at 373 K for 1 h and later calcined at
813 K for 5 h.
La-mordenite and La-beta were prepared by solid cation
exchange adding LaCl3·7H2O (La/Al = 0.33) to 2 g of zeolite at 573 K
for 3 h (samples LaM and LaB). After treatment, samples were fil-
tered and washed several times with deionised water.
Finally, fluorinated mordenite, beta and ZSM-5 samples were
obtained by adding 3.5 mL of NH4F 0.1 M to 1 g of commercial zeo-
lite to have 0.3 wt.% fluorine in the final sample. The slurry formed
was stirred and kept at room temperature for 42 h. Lastly, samples
were calcined at 723 K for 8 h (samples FHM, FHB and FHZ).
Two more beta samples were prepared combining fluorina-
tion and desilication treatments. Sample FHB was desilicated with
NaOH 0.2 M by refluxing at 338 K for 30 min. After treatment, the
sample was filtered, washed and dried overnight to obtain sam-
ple HB(F-DS). The other sample was prepared by desilication of
commercial beta with NaOH 0.2 M by refluxing at 338 K for 30 min.
Then, the sample was cation exchanged with NH4NO3 1 M at 373 K
for 1 h, and later fluorinated at the same conditions as for prepar-
ing sample FHB, resulting in the sample HB(DS-F). Finally, sample
FHB-zeolite A was prepared by physical mixing of sample FHB and
commercial zeolite 4A (Sigma–Aldrich) in a weight ratio of 1/1.
˚
by t-plot analysis of the adsorption data in the 3.5 ≤ t ≤ 5 A t range
by adopting the de Boer reference isotherm equation, whereas pore
size distributions were determined by the Barrett–Joyner–Halenda
(BJH) method.
Infrared spectra were recorded on a Bruker-Equinox-55 FTIR
spectrometer. The spectra were acquired by accumulating 32 scans
at 4 cm−1 resolution in the range of 400–4000 cm−1. Samples were
prepared by mixing the powdered solids with pressed KBr disks
in a ratio of 5:95 and dried in an oven overnight. For adsorbed
pyridine FTIR studies, samples were pressed into self-supported
wafers, and activated at 623 K. Pyridine was adsorbed at 298 K, and
infrared spectra were acquired by accumulating 64 scans at 4 cm−1
resolution in the range of 400–4000 cm−1
.
X-ray photoelectron spectra were taken with a SPECS system
equipped with an Al anode XR50 source operating at 150 W and
a Phoibos 150 MCD-9 detector with pass energy of 25 eV at 0.1 eV
steps at a pressure below 6 × 10−9 mbar.
The acid content of commercial and modified zeolites was mea-
sured using established procedures employing thermal desorption
of cyclohexylamine [26–28]. The method involves thermogravi-
metric analysis (TGA) following adsorption of the base on the
catalysts and determines the number of acid sites capable of inter-
acting with the base after heat treatment at 523 K. Samples were
exposed to liquid cyclohexylamine at room temperature, after
which they were kept overnight (at room temperature) and then in
an oven at 353 K for 2 h so as to allow the base to permeate the sam-
ples [26–28]. The oven temperature was then raised to 523 K and
maintained at that temperature for 2 h in order to remove all the
physisorbed base. Cyclohexylamine desorption TGA curves were
obtained using a Perkin Elmer TGA 7 microbalance equipped with
a programmable temperature furnace. Each sample was heated
from 323 to 973 K at heating rate of 10 K/min under nitrogen flow
(25 mL/min). The weight loss associated with desorption of the base
from acid sites was used to calculate the acid content in mmol of
cyclohexylamine per gram of sample assuming that each mole of
cyclohexylamine corresponds to one mole of protons [26–28].
1H NMR and 27Al NMR spectra were obtained with a Varian Mer-
cury Vx 400 MHz with a probe of 7 mm CPMAS at a frequency of
400 MHz by spinning at 5 kHz. The pulse duration was 2 ␮s and the
One commercial Amberlyst-15 (sample A), supplied by Aldrich
2
˚
(39 m /g, pore size of 103 A, pore volume of 0.34 cc/g, acidity of
4.7 meq H+/g) was also used for comparison.

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M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
0.7
0.6
0.5
0.4
0.3
0.2
0.1
B
HB
DAB
DSHB
LaB
FHB
Fig. 1. XRD patterns for commercial and modified beta samples: (a) B, (b) HB, (c)
DAB, (d) DSHB, (e) LaB, and (f) FHB.
0
20
40
60
80
100
Pore diameter (Å)
delay time was 5 s. The chemical shift reference was trimethyl silil-
3 propionic acid d4-2,2,3,3 sodium salt for 1H NMR, and high purity
aluminium nitrate for 27Al NMR.
Fig. 2. Mesopore size distribution graphics for commercial and modified beta sam-
ples: (a) B, (b) HB, (c) DAB, (d) DSHB, (e) LaB, and (f) FHB.
acidity of H-zeolites was higher than that of Na-zeolites (Table 1),
as expected, due to the presence of H+ compensating the negative
charge of the zeolite framework. 1H NMR spectra of commercial
zeolites showed one peak around 4 ppm (e.g. Fig. 3a), which can be
attributed to free Brønsted protons [32,33]. After protonation, this
peak, related to the protons formed during the treatment, shifted
to higher ppm values (Table 1, e.g. Fig. 3b), as expected [34].
We observed dealumination for all the acid-treated zeolites
(DAM, DAZ and DAB) since they showed higher Si/Al ratio, lower cell
values, and a shift to higher frequencies of the IR bands assigned
to symmetric and asymmetric stretching of the T O bond (T = Si,
Al) than their corresponding Na-zeolites (Table 1). The extent of
dealumination was function of the zeolite structure. Thus, beta
zeolite was easier to dealuminate than mordenite, whereas dea-
lumination of ZSM-5 was very low. This order can be related to
the flexibility of each zeolite framework, and the accessibility of
the aluminium atoms depending on the pores arrangement and
sizes [31,33]. Mordenite and ZSM-5 zeolites are less flexible than
beta, and consequently, it is more difficult to dealuminate them.
Additionally, zeolite beta crystallizes with many stacking faults [35]
while mordenite samples, although less frequently, may also have
structurally related stacking faults [36]. Stacking faults increase the
probability of the presence of defect sites in the framework. Also,
the number of the T-atoms in four-rings may have an influence
on the stability towards dealumination because the tension in the
smaller rings is larger. Thus, a zeolite is easier to dealuminate as
many aluminium atoms has in an environment with tension [31].
The acid and heating conditions used here did not cause drastic
changes in the zeolite structures (e.g. Fig. 1c), although there was
some decrease in the crystallinity of mordenite and beta zeolites
after acid treatment (Table 1). The crystallinity of ZSM-5 samples
practically did not change as a result of their low dealumination.
From nitrogen physisorption results, acid-treated mordenite and
acid-treated ZSM-5 presented higher surface area and lower micro-
pore area/external surface area ratios. This tendency appeared
more marked for the mordenite sample. This can be attributed
to the loss of aluminium in the zeolite structure, which results in
higher mesoporosity, and therefore, higher surface area. However,
after dealumination of beta zeolite, we observed a decrease in the
BET surface area despite the slight higher mesoporosity obtained
(Fig. 2). This can be associated with the loss of crystallinity observed
after treatment, as reported by other authors [37]. Partially dealu-
minated samples showed lower amounts of Brønsted acidity than
their starting zeolites (Table 1), as expected, due to the loss of
2.3. Catalytic activity
Etherification experiments were performed in the liquid phase
in a stainless steel stirred autoclave (150 mL) equipped with tem-
perature controller and a pressure gauge. Stirring was fixed for all
experiments at 1200 rpm to avoid external diffusion limitations.
Typically, the composition of the reaction mixture was: 20 g of glyc-
erol, glycerol/t-butanol molar ratio of 0.25, and constant catalyst
loading of 5 wt.% (referred to glycerol mass). Catalysts were dried
before testing. The reaction temperature used was 348 K. Some
experiments were performed at 363 K, or with a glycerol/t-butanol
molar ratio of 0.125. Samples were usually taken at 1, 3, 6 and 24 h
of reaction. The reaction products were analysed by gas chromatog-
raphy using a chromatograph model Shimadzu GC-2010 equipped
with a SupraWax-280 column and a FID detector.
Glycerol conversion and selectivity to MTBG (glycerol
monoethers) were determined from calibration lines obtained
from commercial products. For DTBG (glycerol diethers) and TTBG
(glycerol triether), which were not available commercially, we
isolated them from the products of the etherification reaction by
column chromatography (1:9 ethyl acetate/hexane) and identified
them by 13C and 1H NMR for proper quantification with the
assistance of the characterization data reported by Jamróz et al.
[29].
3. Results and discussion
3.1. Catalysts characterization
H-zeolites (HM, HZ and HB) maintained the zeolite structure
(see Fig. 1a and b) showing similar crystallinity than their corre-
sponding Na-zeolites (Table 1). HM, HZ and HB had slightly higher
Si/Al ratio than M, Z and B, respectively. Additionally, we observed
a shift to higher frequency values of the IR bands assigned to
symmetric and asymmetric stretching of the T O bond (T = Si, Al)
for these protonated samples (Table 1). The increase of the strength
of the T O bond can be related to a decrease in the Al content since
the Si O bond is shorter than the Al O bond, and Al has lower
electronegativity than Si [30]. These results indicate that some
dealumination occurred for H-zeolites due to the temperature used
during calcination, as reported by other authors [31]. This agrees
with the slight higher mesoporosity and slight higher surface
areas observed for protonated samples (e.g. Fig. 2, Table 1). The

M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
181
Table 1
Characterization of commercial and modified zeolites by XRF, XRD, N2 physisorption, FTIR, TGA and 1H NMR techniques.
Catalyst Si/Al
(XRF)
Crystallinitya Unit cell
(%)
BET areab
(m2/g)
External surface
areab (m2/g)
Micropore area/External
surface areab
IR bands
Acidity capacity
(mmol H+/g)
1H NMR
(ppm)
volumea (Å3)
(cm−1
)
c
ꢁ1
ꢁ2
M
HM
DAM
DSHM
LaM
FHM
6.5
6.9
18.2
4.7
6.5
7.0
100
93
75
62
67
88
2791
2790
2747
2776
2793
2784
303
312
412
319
149
304
31
43
54
45
22
34
8.9
7.1
5.1
6.0
1.6
8.1
1068
1075
1093
1075
1069
1070
629
630
646
636
627
628
0.10
0.18
0.08
0.15
0.16
0.11
4.6
6.1
5.1
4.4
6.4
6.5
B
HB
DAB
DSHB
LaB
FHB
10.0
11.0
110.7
4.2
10.1
11.8
100
89
61
56
44
82
–
–
–
–
–
–
573
579
554
663
384
496
203
218
197
304
133
142
1.8
1.7
1.5
1.2
1.5
1.7
1068
1086
1091
1078
1068
1073
629
633
641
630
628
629
0.41
0.60
0.18
0.50
0.55
0.46
4.2
4.4
4.0
3.9
4.5
4.7
Z
HZ
DAZ
DSHZ
FHZ
20.0
20.2
22.3
14.2
20.3
100
100
99
97
100
5209
5186
5172
5122
5154
300
307
334
353
305
87
114
127
135
110
2.4
2.3
2.1
1.6
1.8
1063
1068
1096
1073
1065
797
797
797
795
797
0.25
0.37
0.21
0.28
0.26
3.8
4.2
4.0
3.7
4.8
a
Calculated from XRD patterns.
Calculated from N2 physisorption results.
Frequencies of the main asymmetric stretch (ꢁ1), and the main symmetric stretch (ꢁ2) due to the T O bond (T = Si, Al).
b
c
extraframework cations as a consequence of the dealumination.
1H NMR spectra of DAM and DAZ presented one broad peak at
higher ppm values than that observed for M and Z, respectively
(Table 1). This peak could be assigned to Brønsted protons, formed
by cation exchange during dealumination in HCl medium, which
are interacting with the zeolite framework [31,33]. For DAB, we
observed one peak in the 1H NMR spectrum (Fig. 3c) with a shift
similar to that of commercial beta (Fig. 3a). This can be explained by
the higher dealumination suffered by this sample. Therefore, there
are no protons interacting with the framework since practically the
zeolite framework has no negative charge.
All basic-treated and later protonated samples (DSHM, DSHB,
DSHZ) showed desilication since they had lower Si/Al ratios and
lower cell volume values than commercial zeolites (Table 1). The
frequencies of the IR bands assigned to symmetric and asym-
metric stretching of the T O bond (T = Si, Al) had similar values
(Table 1) to those reported by other authors for desilicated zeo-
lites [38]. Additionally, some dealumination cannot be discarded
due to the temperature used during calcination in the last step
of protonation. Desilication was higher for beta than for ZSM-5
and mordenite. This can be related to the low stability of frame-
work aluminium in beta compared to ZSM-5 and mordenite taking
into account that lattice aluminium controls silicon extraction of
the zeolite framework [39]. The zeolite structure maintained after
desilication-protonation treatment (e.g. Fig. 1d), although some
decrease in the crystallinity of mordenite and beta was observed
(Table 1). Desilicated-protonated samples showed higher surface
area and higher mesoporosity (e.g. Fig. 2) than their corresponding
starting zeolites, as expected, due to the loss of silicon in the zeo-
lite structure. Interestingly, acidity was slightly higher (Table 1) but
the peak observed in the 1H NMR spectrum shifted to lower ppm
values (Table 1, e.g. Fig. 3d) for the desilicated samples than for
the commercial ones. The slight higher acidity can be explained by
the second step of protonation applied to desilicated samples that
led to the presence of H+ in these modified zeolites whereas the
less strength of these acid sites could be related to variations in the
zeolite composition due to the treatment.
for these samples (Table 1). XRD patterns of LaB and LaM showed
the maintenance of the zeolite structure (e.g. Fig. 1e) although a
considerable decrease of crystallinity was observed for both sam-
ples (Table 1). The lower surface areas accompanied by some
decrease of porosity (e.g. Fig. 2) observed for La-modified samples
can be attributed to the presence of bulky hydrolysed lanthanum
cations (La(OH)2+) blocking the pores. The introduced hydrolysed
lanthanum cations led to similar amount of acid sites than their cor-
responding H-zeolites (Table 1) but with slightly stronger Brønsted
acidity (a shift to higher ppm values was observed in the 1H NMR
spectra, e.g. Fig. 3e) according to the results reported by other
authors [40,41]. The lanthanum exchanged in the zeolites can cause
a polarization of the zeolite framework increasing the strength of
the Brønsted acid sites [42].
Zeolite modification by treatment with NH4F practically did not
affect the zeolite structure (e.g. Fig. 1f), except for some decrease
in the crystallinity detected for FHM and FHB (Table 1). In addition,
the Si/Al ratio, the position for the symmetric and asymmetric TO4
tetrahedra bands in the mid-IR region (Table 1) and the nitrogen
adsorption–desorption isotherm shapes of the fluorinated samples
were very similar to those observed for their corresponding Na-
zeolites (Table 1, e.g. Fig. 2). The presence of fluorine in the zeolite
framework was confirmed by XPS. F1s XP spectra of fluorinated
samples showed one peak with two components: one at ca. 685
with much higher area than the other appeared at ca. 687 eV (e.g.
Fig. 5). These two peak components can be assigned to fluoride
species interacting with Al atoms, and with Si atoms, respectively
(Scheme 1) according to the results reported by other authors for
fluorinated zeolites [43]. The formation of fluorocomplexes of Al
by the introduction of fluorine in the zeolite framework has been
related to an increase in the acidity of the Brønsted acid sites
through an inductive effect by fluorine whereas the formation of
fluorocomplexes of Si has been correlated with a decrease of the
Brønsted/Lewis acidity ratio [43]. Therefore, the higher contribu-
tion of the component appeared at 685 eV observed for our fluori-
nated samples indicated the incorporation of fluorine in the zeolite
framework. 27Al NMR spectra of fluorinated samples were very
similar to those of the starting zeolites (e.g. Fig. 4). This indicates
that fluorination at mild conditions did not cause appreciable dea-
lumination. The signals corresponding to tetrahedral and octahe-
dral aluminium appear around 50 ppm and 0 ppm, respectively. The
La-containing samples exhibited similar Si/Al ratio than their
corresponding commercial zeolites (Table 1). This agrees with the
similar frequencies values of the IR bands assigned to symmetric
and asymmetric stretching of the T O bond (T = Si, Al) obtained

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M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
Fig. 3. 1H NMR spectra of the samples (a) B, (b) HB, (c) DAB, (d) DSHB, (e) LaB, and (f) FHB.
presence of octahedral Al in commercial beta can be attributed to
extraframework aluminium species or to aluminium coordinated in
defect sites taking into account the characteristic stacking faults of
this zeolite structure [35,44]. Acidity capacity values of fluorinated
samples were slightly higher than those of Na-zeolites (Table 1).
Additionally, from 1H NMR results, we observed that, after fluori-
nation, the peak attributed to Brønsted protons that are interacting
with the zeolite framework, shifted to higher ppm values (e.g.
Fig. 3f). This only can be explained by an inductive effect by F con-
firming the introduction of fluorine atoms in the zeolite framework.
3.2. Catalytic activity
Table 2 shows the catalytic activity results obtained for all
zeolite catalysts for the etherification reaction of glycerol with
tert-butanol after 24 h of reaction. One acid ion-exchange resin
(Amberlyst 15, here named as A), which is a typical acid catalyst
used for this reaction, has been also tested at the same reaction
conditions for comparison. The reaction products obtained were
mono-tert-butyl glycerol ether (MTBG), di-tert-butyl glycerol ether
(DTBG) and sometimes low amounts of tri-tert-butyl glycerol ether
(TTBG). No other reaction products were detected in any case.
Scheme 1. Fluorination mechanism of zeolites: (a) formation of fluoride species
interacting with Al atoms, and (b) formation of fluoride species interacting with Si
atoms.

M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
183
Fig. 4. 27Al MAS NMR for (a) NaM, (b) FHM, (c) NaB, and (d) FHB samples.
On the whole, independently on the modifications performed,
beta catalysts were more active than ZSM-5 and mordenite cat-
alysts in this order (Table 2). This behaviour was previously
observed, specifically when the same commercial Na-zeolites used
in this study were tested for the isomerization of styrene oxide
to ␤-phenylacetaldehyde, a reaction catalysed by Brønsted acid
sites [33]. The differences in the activity between the catalysts
prepared from the three types of zeolites could be explained by
the number of Brønsted acid sites and their strength (Table 1)
together with the accessibility of the reactants to the active
sites. Beta and ZSM-5 samples had much higher external surface
area and higher acid capacity than mordenite samples (Table 1)
despite to have a three-dimensional pore structure compared
with the one-dimensional pore structure of mordenite. Thus, zeo-
lite beta has a three-dimensional 12-ring pore system (straight
˚
˚
channels of diameter 6.6 A × 6.7 A and sinusoidal channels of diam-
˚
˚
eter 5.6 A × 5.6 A), zeolite mordenite has a one-dimensional pore
´
˚
˚
system with main channels of diameter 6.7 A × 7.0 A and com-
˚
˚
pressed channels of diameter 2.6 A × 5.7 A whereas ZSM-5 has a
three-dimensional 10-ring pore system with channels of diameter
´
˚
˚
5.1 A × 5.5 A. Additionally, it is important to note that morden-
ite is the most hydrophilic structure of the three zeolites used in
this study since it has the lowest Si/Al ratio (Table 1). Taking into
account that water is formed during reaction, a major competition
between water and the reactants for the active sites can be expected
for this zeolite. All these factors could explain the lower conversion
values obtained for mordenite catalysts.
Table 2
Catalytic activity of Amberlyst-15, commercial and modified mordenite, beta and
ZSM-5 catalysts.
Catalyst
Conversion (%)
MTBG (%)
h-GTBE (%)
Regarding the catalytic results of Beta samples (Table 2), we can
observe that Na-Beta showed lower conversion and lower selectiv-
ity to high-glycerol tert-butyl ethers (h-GTBE) than Amberlyst-15
(catalyst A). This can be related to the lower amount and strength
of acid sites of Na-Beta compared with Amberlyst-15, a macropo-
rous acid ion-exchange resin with sulfonic groups. The activity of
commercial Na-zeolites can be attributed to the presence of free
Brønsted protons, as detected by 1H NMR [32,33]. When B was
A
M
81
10
29
30
27
8
64
77
89
78
100
95
36(1)
23
11
22
0
5
0
9
HM
HMa
DAM
DSHM
LaM
FHM
23
32
100
81
B
63
83
66
71
29
61
16
75
74
74
66
70
100
82
26
26
34
30
0
18
22
37(1)
Bb
HB
HBc
DAB
DSHB
LaB
FHB
78
63
Z
HZ
DAZ
DSHZ
FHZ
35
58
22
22
33
100
96
100
97
0
4
0
3
8
92
Reaction conditions: 5.0 wt.% of catalyst referred to glycerol mass, glycerol/t-
butanol molar ratio = 0.25, reaction temperature = 348 K, reaction time = 24 h, stir-
ring = 1200 rpm. MTBG: glycerol monoethers; h-GTBE: glycerol diethers + glycerol
triether. In parenthesis, selectivity to glycerol triether (%).
695
691
687
683
679
675
a
Reaction time = 48 h.
Glycerol/t-butanol molar ratio = 0.125.
Binding Energy (eV)
b
c
Glycerol/t-butanol molar ratio = 0.125 and reaction temperature = 363 K.
Fig. 5. Deconvoluted F 1 s XP spectrum of the sample FHB.

184
M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
tested at lower glycerol/t-butanol ratio, conversion increased and
was comparable to that of Amberlyst-15 but the products selectiv-
ity values did not change (Table 2). H-Beta led higher conversion
and higher selectivity to h-GTBE than Na-Beta, as expected, tak-
ing into account the higher amount and strength of its Brønsted
acid sites (Table 1). When HB catalyst was tested at higher reaction
temperature (363 K) and lower glycerol/t-butanol ratio, conver-
sion increased but again the selectivity to h-GTBE did not improve
(Table 2). One important feature to remark is that triether was
not detected for Na-Beta or H-Beta. This has been attributed by
other authors to steric hindrance effects because of the micro-
porosity of the zeolites [14]. Partially dealuminated beta (DAB)
presented lower conversion than its corresponding Na- and H-beta,
and null selectivity to h-GTBE (Table 2). This can be related to the
lower amount of acid centres, which also had slight lower acid-
ity, observed for DAB due to dealumination (Table 1). This reveals
the importance of acidity for this reaction since the additional
mesoporosity generated during dealumination was not enough to
favour the formation of the high-glycerol tert-butyl ethers (di- and
triether). Desilicated-protonated Beta (DSHB) also showed lower
conversion and lower selectivity to h-GTBE than B or HB (Table 2).
This was surprising since this catalyst had slight higher acidity
capacity and higher mesoporosity to avoid possible steric hin-
drance than those previously discussed (Table 1). These results can
be explained by the lower strength of the Brønsted acid sites of
desilicated samples, as observed from 1H NMR (Table 1). There-
fore, the acid strength of the catalytic sites influences significantly
the catalytic performance, both conversion and selectivity to h-
GTBE. La-containing beta catalyst showed the lowest conversion
of all beta catalysts without improving the selectivity to h-GTBE
(Table 2). The presence of bulky hydrolysed lanthanum cations
(La(OH)2+) blocking the pores increased steric hindrances despite
the higher strength of the Brønsted acid sites of this catalyst
(Table 1). Thus, the accessibility of the reagents to the catalytic
sites strongly affects the catalytic results. Finally, fluorination of
beta zeolite at mild conditions resulted in a catalyst, which yielded
the best conversion and selectivity to h-GTBE due to the higher
amounts of the strongest Brønsted acid sites generated because
of the incorporation of fluorine in the zeolite framework. Inter-
estingly, FHB allowed us to detect the presence of tri-tert-butyl
glycerol ether (TTBG) in low amounts. This is the first time that the
formation of glycerol triether in a zeolite, for the glycerol etheri-
fication with tert-butanol, has been reported. Taking into account
that this catalyst has similar porosity than NaB or HB (Table 1 and
Fig. 2), we can conclude that the higher acidity strength achieved
by incorporating low amounts of fluorine in the zeolite structure is
fundamental to favour the production of the bulky triether. Addi-
tionally, catalytic activity of this catalyst was comparable to that
obtained with Amberlyst-15 (Table 2).
Fig. 6. Conversion vs acidity amount × acidity strength, and selectivity to h-GTBE vs
acidity strength graphs for beta catalysts. Reaction conditions: 5.0 wt.% of catalyst
referred to glycerol mass, glycerol/t-butanol molar ratio = 0.25, reaction tempera-
ture = 348 K, stirring = 1200 rpm.
surface area, more hydrophilic character and lower dimensional-
ity of the mordenite structure, as previously commented, whereas
the selectivity to h-GTBE could be attributed to its higher acid-
ity strength (Table 1). After protonation, conversion increased, as
expected due to the presence of higher amounts of acid centres
with higher acidity strength but the selectivity to h-GTBE was lower
(Table 2). This tendency was also observed for the partially dealu-
minated mordenite (DAM) and for fluorinated mordenite (FHM).
These catalysts had similar amount of acid sites but with stronger
acidity than Na-mordenite (Table 1). The low external surface area
and the lower dimensionality and higher hydrophilic character
of the mordenite structure could affect the subsequent consec-
utive reactions from the higher amounts of monoether formed
initially on these most active mordenite catalysts. Probably, longer
reaction times should be required to favour the equilibrium for
the formation of di- and triethers. Actually, when HM was tested
for 48 h of reaction, higher selectivity to h-GTBE was achieved
(Table 2). Desilicated-protonated mordenite showed lower con-
version and lower selectivity to h-GTBE than Na-mordenite. This
can be explained by the slight lower acidity strength of its acid
sites. The incorporation of lanthanum in mordenite did not improve
the conversion values obtained with HM or DAM and yielded total
selectivity to the undesired glycerol monoethers (Table 2). This cat-
alytic behaviour was previously observed for LaB. The presence
of bulky hydrolysed lanthanum cations (La(OH)2+) blocking the
pores increased steric hindrances despite the higher strength of
the Brønsted acid sites of this catalyst (Table 1).
All ZSM-5 catalysts showed moderate conversion values and
very low or null selectivity to h-GTBE (Table 2). From these cat-
alytic results, we can conclude that the modifications made on this
zeolite by the different treatments did not affect considerably its
surface and acidic properties (Table 1) and, consequently, its cat-
alytic behaviour. The most interesting result is that fluorination of
ZSM-5 at mild conditions, allowed to obtain a catalyst (FHZ), which
led to slight higher selectivity to h-GTBE than the rest of ZSM-5
catalysts. This only can be related to the generation of stronger
Brønsted acid sites (Table 1), as previously observed for fluorinated
beta catalyst, because of the incorporation of fluorine in the zeolite
framework.
Mordenite catalysts showed a catalytic behaviour very different
when compared with ZSM-5 and beta catalysts. Commercial Na-
mordenite showed low conversion but comparable selectivity to
h-GTBE than commercial Na-beta (Table 2). The lower conversion
could be related to the lower amount of acid centres, lower external
Fig.
6
represents conversion vs acidity amount × acidity
strength, and selectivity to h-GTBE vs acidity strength for beta

M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
185
100
90
80
70
60
50
40
30
20
A
B
HB
DAB
DSHB
LaB
FHB
10
0
0
3
6
9
12
15
18
21
24
Reaction time (h)
Fig. 7. Study of the variation of conversion with time for beta catalysts. Reaction
conditions: 5.0 wt.% of catalyst referred to glycerol mass, glycerol/t-butanol molar
ratio = 0.25, reaction temperature = 348 K, stirring = 1200 rpm.
catalysts. As we can observe, there is quite good lineal correla-
tion between conversion with respect to acidity amount × acidity
strength, except for one of the points which corresponds to the LaB
sample. This confirms that the amount and strength of Brønsted
Fig. 8. FTIR spectra of adsorbed pyridine for FHB, FHB-AR (after reaction) and FHB-
AEXT (after extraction). B: Brønsted acid sites; L: Lewis acid sites.
acid sites strongly influences conversion values. For LaB sample,
+
the presence of bulky hydrolysed lanthanum cations (La(OH)2
)
1500
blocking the pores increased steric hindrances despite the higher
strength of the Brønsted acid sites of this catalyst, as commented
above. Interestingly, selectivity to h-GTBE vs acidity strength also
showed good correlation, indicating that acidity strength mainly
affects selectivity to di- and tri-ethers. For mordenite and ZSM-5
samples, representation of conversion vs acidity strength × acidity
amount and selectivity to h-GTBE vs acidity strength (not shown
here) did not exhibit clear correlations in any case. This means, that
other factors, such as the accessibility or diffusion of the reactants
to the acid sites, also influences conversion and selectivity values
for these catalysts.
1000
500
b
a
0
5
10
20
30
2-Theta - Scale
Fig. 9. XRD patterns of catalyst FHB: (a) before reaction and (b) after reaction.
Fig. 7 shows the variation of conversion with time for Amberlyst
and beta-zeolite catalysts. Conversion of Amberlyst increased from
1 to 6 h of reaction, but after this time, maintained practically con-
stant. In contrast, for NaB, HB, DSHB and FHB catalysts the increase
of conversion with time was more gradual and continuous even
after 6 h of reaction. Lastly, conversion values of catalysts DAB and
LaB practically did not change with the reaction time. The vari-
ation of activity with time of these catalysts was accompanied
by a decrease of their surface areas (Table 3) and a considerably
decrease of their Brønsted acid sites (e.g. Fig. 8). This indicates a
loss of active centres during reaction, and therefore some catalysts
deactivation. However, XRD did not show significant changes in the
zeolite structure after reaction in any case except for some decrease
in crystallinity (e.g. Fig. 9). From these results, we can conclude that
there is no collapse of the structure during reaction, but blocking
of pores by reaction products. One of the problems of this reaction
is that the formation of water during reaction inhibits the glycerol
etherification since water competes with tert-butanol and glycerol
for the active site adsorption [24]. The more gradual variation of
conversion with time observed for NaB, HB, DSHB and FHB with
respect to the acid ion-exchange resin can be related to the nature
of the zeolite, which can partially adsorb the water formed during
reaction in contrast with Amberlyst, which was strongly affected
by the presence of water, as reported by other authors [24]. The
almost no variation of conversion observed for DAB and LaB cata-
lysts can be explained by the low amount of acid centres of catalyst
+
DAB, and because of the effect of bulky La(OH)2 cations blocking
the pores for LaB.
Table 4
Catalytic activity of beta catalysts after 6 h of reaction.
Table 3
Catalyst
Conversion (%)
MTBG (%)
h-GTBE (%)
Characterization of several beta-zeolite catalysts before reaction, after 24 h of reac-
tion, and after extraction.
B
HB
DAB
DSHB
LaB
FHB
53
55
32
35
21
56
73
70
100
82
80
77
27
30
0
18
20
23
Catalyst
BET area before
reaction (m2/g)
BET area after
BET area after
reaction (m2/g)
extraction (m2/g)
B
HB
DAB
DSHB
LaB
FHB
573
579
554
663
384
496
361
364
99
124
45
457
429
387
498
276
470
Reaction conditions: 5.0 wt.% of catalyst referred to glycerol mass, glycerol/t-
butanol molar ratio = 0.25, reaction temperature = 348 K, reaction time = 6 h, stir-
ring = 1200 rpm. MTBG: glycerol monoethers; h-GTBE: glycerol diethers + glycerol
triether. In parenthesis, selectivity to glycerol triether (%).
334

186
M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
Fig. 10. Comparison of conversion and selectivity to the three-glycerol ethers with the reaction time for catalysts FHB and FHB mixed with zeolite A. Reaction conditions:
5.0 wt.% of catalyst referred to glycerol mass, glycerol/t-butanol molar ratio = 0.25, reaction temperature = 348 K, reaction time = 24 h, stirring = 1200 rpm. MTBG: glycerol
monoethers; DTBG: glycerol diethers.
Regarding selectivity values, Table 4 shows the results obtained
for beta catalysts after 6 h of reaction. On the whole, at lower reac-
tion times selectivity to h-GTBE was similar or slight lower than
after 24 h of reaction. Therefore, higher reaction times favoured
slightly the obtention of higher amounts of h-GTBE. Interestingly,
glycerol triether was not observed for catalyst FHB at lower reaction
times, even after 12 h or 18 h of reaction (not shown here).
In order to evaluate the nature of the products remaining in
the catalytic pores after reaction, several used catalysts, after fil-
tration, were submitted to extraction by refluxing with 50 mL of
ethanol for 1 h. After rotary evaporation of the solvent, the resulting
solution was analysed by gas chromatography. All chromatograms
showed the presence of several peaks corresponding to glycerol and
products of reaction (monoethers and diethers in less proportion).
Interestingly, surface areas and the amount of Brønsted acid sites of
the catalysts were partially recovered after extraction (Table 3, e.g.
Fig. 8). Therefore, the presence of reagents and reaction products
in the pores decreases the accessibility of the reagents to the acid
sites.
From all these results, we tried to improve the selectivity
towards the triether detected with FHB. Thus, four new fluori-
nated beta catalysts were synthesized at different conditions and
tested for this reaction. Two samples were prepared by the same
method as FHB but increasing the theoretical fluorine content to
obtain 1 wt.% F (FHB(1%)) and 10 wt.% F (FHB(10%)), with the idea
to increase the number of stronger acid sites. The other two samples
were synthesized by combining desilication and fluorination pro-
cedures (HB(DS-F) and HB(F-DS)), as described in Section 2, with
the aim to increase acidity and mesoporosity at the same time. The
catalytic results of these four new catalysts are shown in Table 5.
As we can observe, the increase in the percentage of theo-
retical fluorine did not improve the catalytic results obtained for
sample FHB. This suggests that not all the theoretical fluorine
was introduced in the zeolite structure and some framework dea-
lumination occurred for both samples (especially for FHB(10%)).
Dealumination was confirmed (not shown here) by the decrease
of the cell volumes, the gradual increase of octahedral aluminium
observed by 27Al NMR, the higher mesoporosity and the shift to
higher frequencies of the IR bands, assigned to symmetric and
asymmetric stretching of the T O bond (T = Si, Al) of these two
samples when comparing with the starting zeolite. Therefore, fluo-
rination treatment with higher amounts of fluorine did not increase
significantly the introduction of fluorine in the structure, and for
this reason, the catalytic results of these catalysts were very similar
to those of FHB.
Si/Al ratios, determined by XRF, were 4.0 and 4.1 for HB(DS-
F) and HB(F-DS), respectively, confirming desilication. The sample
first desilicated and later fluorinated had less wt.% fluorine (0.1)
than the sample first fluorinated and later desilicated (0.25) as
determined by chemical analysis. This explains the catalytic results
obtained since catalyst HB(DS-F) showed lower conversion and
lower selectivity to h-GTBE whereas catalyst HB(F-DS) had sim-
ilar conversion and slight lower selectivity to h-GTBE than FHB
(Table 2). Interestingly, low amounts of tri-tert-butyl glycerol ether
(TTBG) were detected for all four catalysts. This confirms the influ-
ence of the introduction of fluorine in the zeolite structure on the
acidity, and therefore, the significant effect of the stronger Brønsted
acidity, generated by fluorination, on the formation of the triether,
independently of the porosity.
Table 5
Catalytic activity results of several fluorinated beta catalysts for the glycerol ether-
ification with tert-butanol.
Catalyst
Conversion (%)
MTBG (%)
h-GTBE (%)
FHB(1%)
FHB(10%)
HB(DS-F)
HB(F-DS)
72
70
51
74
64
66
90
70
36 (1)
34 (1)
10 (<1)
30 (1)
Reaction conditions: 5.0 wt.% of catalyst referred to glycerol mass, glycerol/t-
butanol molar ratio = 0.25, reaction temperature = 348 K, reaction time = 24 h, stir-
ring = 1200 rpm. MTBG: glycerol monoethers; h-GTBE: glycerol diethers + glycerol
triether. In parenthesis, selectivity to glycerol triether (%).

M.D. González et al. / Applied Catalysis A: General 450 (2013) 178–188
187
As commented above, one of the problems of this reaction is the
formation of water during reaction [24]. The higher water acidity
in relation to tert-butanol results in a lowering of catalyst activity
due to the formation of solvated sites, as reported in the litera-
ture [45,46]. Frusteri et al. proposed that the removal of water
from the reaction medium could increase the formation of high
glycerol ethers [24]. These authors performed one experiment by
stopping the etherification reaction after 6 h, dehydrated the reac-
tion mixture by zeolites, and then continued with the reaction for
6 more hours. The results showed an important increase in the for-
mation of diethers [24]. From this idea, we designed one similar
catalytic experiment using a mixture of sample FHB and commer-
cial zeolite 4A in a weight ratio of 1/1 as catalyst. Zeolite A was
inactive for this reaction. Fig. 10 shows the comparative evolution
with time of conversion and selectivity towards the three-glycerol
ethers for catalysts FHB and FHB-zeolite A. After 1 h of reaction,
the catalytic results were just slightly better for the catalyst mixed
with the molecular sieve. This improvement in the conversion and
selectivity to h-GTBE were more marked at higher reaction times.
This confirms that the presence of zeolite A in the medium helped
to adsorb the water generated during the etherification reaction
allowing a better approach of the reagent molecules to the Brønsted
acid sites of catalyst FHB, and therefore favouring the formation of
high ethers. We also observed a higher slight increase of the tri-
ether amount with time for the catalyst mixed with zeolite A (not
shown here).
From all these results, we can establish that the amount
and strength of Brønsted acid sites affects conversion whereas
the acidity strength significantly influences the formation of di-
and tri-ethers of glycerol. However, the accessibility of the reac-
tants to the acid sites must be guaranteed so that they can
act.
Acknowledgments
The authors are grateful for the financial support of the
Ministerio de Ciencia e Innovación and FEDER funds (CTQ2008-
04433/PPQ). Dolores González acknowledges Ministerio de Edu-
cación y Ciencia for a FPU grant (AP2007-03789).
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4. Conclusions
On the whole, beta catalysts were more active than ZSM-5 and
mordenite catalysts in this order for the etherification of glycerol
with tert-butanol. This could be related to the number of Brønsted
acid sites and their strength together with the accessibility of the
reactants to the active sites.
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lower external surface area together with the lower dimensionality
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